Eye tracking refers to the process of determining eye movement and/or gaze point. Eye tracking is widely used, for example, in psychology and neuroscience, medical diagnosis, marketing, product and/or user interface design, and human-computer interactions. Some eye tracking methods use search coils, electrooculogram, or special contact lens. These methods are considered to be intrusive as the eye trackers need to be in contact with the subject's eyes or the face regions surrounding the eyes.
Non-contact optical methods also have been developed for eye tracking. For example, light (e.g., infrared) can be directed to, and reflected by, the eye and sensed by a video camera or other optical sensor. The images or signals are analyzed to extract eye movement information. Video-based eye trackers usually utilize the corneal reflection and the center of the pupil, although some optical methods for eye tracking utilize image features from inside the eye, such as the retinal blood vessels. Corneal reflection and pupil center based eye tracking methods are widely used because they are non-invasive and relatively inexpensive.
Eye tracking methods also can be divided into two categories: head-mounted and remote. Head-mounted eye trackers are worn on the subject's head, whereas remote eye trackers are placed at a certain distance from the subject, for example, on a table or computer display.
The present disclosure relates to remote eye tracking, utilizing corneal reflection and pupil center. It is assumed there is a target screen plane at which the subject is looking (e.g., television screen, computer screen, electronic tablet or smart phone display).
FIG. 1A illustrates an example of a remote eye tracker utilizing corneal reflection and pupil center. Each illuminator 20 forms a bright spot in the image of the eye obtained by the camera 22 as a result of corneal reflection (see FIG. 1B). Each illuminator 20 can comprise a light source and may consist of multiple light emitting elements placed next to each other. If the subject's head is at a fixed pose, the direction of the subject's gaze can be determined by the vector formed between the center of corneal reflection and the center of pupil (see “OG” in FIG. 1(c)), which can be mapped to a target screen. Thus the coordinates of the gaze point (G) can be determined using the horizontal and vertical components of the gaze direction and its distance from the subject's eye 24.
Using two or more illuminators 20 in video eye trackers can be advantageous for several reasons. First, when there is head movement during eye tracking, the gaze direction is dependent on the head pose, in addition to the pupil center and corneal reflection. Multiple corneal reflections from the illuminators provides additional information from which head pose can be determined, and thus allow head movements during eye tracking.
Further, the separation between the corneal reflections appearing on the image (FIG. 1B) sensed by the camera 22 is a function of the distance (D) between the eye and the eye tracker. For a given camera-illuminator configuration, the corneal reflection separation in the image becomes smaller when the eye is further away from the eye tracker. Thus the eye to eye tracker distance (D) can be estimated from the corneal reflection separation. In general, as the eye to eye tracker distance (D) increases, the corneal reflection separation becomes less sensitive (i.e., diminishing differentiation power). In such instances, it is necessary to increase the illuminator separation (L) for larger eye to eye tracker distance (D). The illuminator separation (L), however, is one of the main factors affecting the size (e.g., length) of a remote video eye tracker.